Single ended vs differential signals -- Plug-in data acquisition boards

As previously demonstrated, great care must be taken in the connection, earthing and shielding of signals, received from external transducers (or similar signal sources), to signal conditioning equipment. This is especially true where the signal levels are very small and /or the signal sources are a long way from the measuring equipment. In these cases, the effects of earth loops, induced noise, and common mode voltages can introduce errors that lead to large inaccuracies in the signal measurement.

The three basic configurations for connecting input signals to signal conditioning equipment are available on plug-in A/D boards:

• Single ended

• Pseudo differential

• Differential

Single ended inputs

Single ended inputs are those where the signal is transmitted over a single conductor and referenced to analog ground AGND. The single conductor is connected to the HI terminal of the amplifier while the LO terminal of the amplifier is connected to AGND. This is shown in ill. F.10.

ill. F.10 Single ended inputs

Single ended inputs usually carry high-level signals (in the order of volts), which don't require high gains (> × 5), transmitted over short distances (0.5 m). Where they are required to be transmitted over longer distances, they should be shielded, and the shield connected to AGND at the instrument end only.

While this configuration allows more inputs to be multiplexed to a single A/D converter, it should only be used where there is no practical way of bringing a remote ground or an analog ground back to the measurement point.

Because the amplifier LO terminal is connected to AGND, what is amplified is the difference between Esn + Vcm and AGND. This introduces the common mode offset voltage as an error.

Plug-in boards that don't have an amplifier (i.e. where the multiplexed input is fed straight to the A/D converter) must use the single ended input configuration.

Pseudo-differential configuration

The pseudo-differential input configuration is a variation of the single ended input con-figuration, providing some degree of common mode rejection while still allowing the maximum number of multiplexed input channels.

ill. F.11 Pseudo differential inputs

In the configuration shown in ill. F.11 the signal source LO outputs are all connected to the LO input terminal of the amplifier, while the signal source HI outputs are connected to the respective HI input for each of the channels. The LO input to the amplifier is then referenced to analog ground AGND at the signal end of the cable. This method is only possible if the LO terminal of the amplifier is brought out to the connector and the signal sources can be grounded at their signal ends. In using this configuration only the difference between the channel input Esn + Vcm and the signal ground, which has the common mode voltage on it, is amplified.

Differential inputs

True differential inputs, where the HI and LO outputs of the signal source are connected directly to the HI and LO terminals of the amplifier, as shown in ill. F.12, offer the greatest noise immunity and common mode rejection.

ill. F.12 Differential input configuration

In this configuration, only the difference in the signal inputs is measured. Noise induced equally in each signal line will cancel out at the inputs of a true balanced differential amplifier, while common mode voltages (appearing at both inputs) will be rejected if the amplifier has a large CMRR. This should not preclude the added precautions of twisting differential pairs and providing earthed shields to reduce noise induced in long cables.

Differential inputs should be used:

• When measuring signals with large common mode voltages (e.g. strain gauges).

• Where several transducers with different ground points (and possibly different ground potentials) are to be measured. Connecting the Lo channel of each sensor together at a common point, as in the pseudo differential connection, can create unwanted ground currents that induce offset and noise errors at the amplifier inputs.

• When measuring signal voltages that are very small, and the signal/noise ratio is low.

• When the input transducer is physically located a large distance from the measuring device and may be susceptible to the effects of noise.

Note that for differential input configuration, two input multiplexers are needed, and for the same number of input terminals as single ended and pseudo differential inputs, half the number of input channels is available in differential mode.

Where high impedance sources are used, bias resistors may be required to return bias currents to the operational amplifier, thus preventing the floating of inputs beyond the limits of the amplifier inputs. Such bias resistors normally consist of high impedance resistors, typically between 100 K ohm and 1 M ohm , connected between the HI and LO signal lines and AGND.

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